Question: Which of these numbers is composite? ${13,\ 23,\ 43,\ 83,\ 91}$
A composite number is a number that has more than two factors (including 1 and itself). 13, 23, 43, and 83 each have only two factors. The factors of 91 are 1, 7, 13, and 91. Thus, 91 is the composite number.